Antiphospholipid syndrome (APS) is an autoimmune disease characterized clinically by thrombosis and/or recurrent fetal loss. Beta2-glycoprotein I (2GPI) is the major antigen for the antibodies associated with APS. Only the dimeric form of 2GPI generated by anti-2GPI antibodies is pathologically important, in contrast to monomeric 2GPI which is abundant in plasma. The primary hypothesis of the proposed study is that the interaction of 2GPI/anti-2GPI antibody complexes with lipoprotein receptors and anionic phospholipid is a potential drug target for interfering with thrombosis in APS. We created a dimeric inhibitor to selectively target dimeric 2GPI in 2GPI/anti-2GPI antibody complexes. This inhibitor disrupts the binding of 2GPI/antibody complexes with lipoprotein receptors and anionic phospholipid. The proposed studies will optimize the inhibitor that blocks the interaction of 2GPI/anti-2GPI antibody complexes with lipoprotein receptors and anionic phospholipid; use the inhibitor, site-directed mutagenesis, and functional studies in cells and in vivo to dissect the contribution of lipoprotein receptors and anionic phospholipid to thrombosis in APS. The studies will employ biophysical measurements of domain interactions, NMR and crystallographic determination of protein structures, cell-based assays and laser-induced thrombosis in live mice in pursuit of these goals. PUBLIC HEALTH RELEVANCE: Beta2-glycoprotein I (2GPI) is the major antigen for the antibodies associated with antiphospholipid syndrome (APS), an autoimmune disease characterized by thrombosis and recurrent pregnancy loss. APS patients with thrombosis are treated chronically with general antithrombotic medications which are not always effective. We will use in vivo mouse model in combination with biochemical and biophysical studies to understand how 2GPI/anti-2GPI antibody complexes interact with cellular receptors leading to thrombosis in APS. The insights gained from the study could be used as a basis for new treatments for people suffering from APS.